1. Field of the Invention
The invention relates to an image forming apparatus and an elastic roller.
2. Related Background Art
Hitherto, in image forming apparatuses such as printer of an electrophotographic system, copying apparatus, facsimile apparatus, and the like, a surface of a photosensitive drum is uniformly and evenly charged by a charging roller and exposed by an exposing apparatus, an electrostatic latent image is formed onto the surface, the electrostatic latent image is developed by a developing roller, and a toner image is formed. The toner image is transferred onto a print medium by a transfer roller, the transferred toner image is fixed by a fixing apparatus, and an image is formed (for example, refer to JP-A-9-212012).
The transfer of the toner image is executed in a nip portion which is formed between the photosensitive drum and the transfer roller. For this purpose, a potential difference is formed between the photosensitive drum and the transfer roller and toner on the photosensitive drum is electrostatically moved onto the print medium by the potential difference. Therefore, to improve image quality, it is important to move the just enough toner on the photosensitive drum onto the print medium.
That is, if the potential difference is too large, the transfer becomes excessive, the toner on the photosensitive drum is moved at a position just before the position on an upstream side of the nip portion in the conveying direction of the print medium and defective printing such as what is called dust printing occurs. On the contrary, if the potential difference is too small, the transfer is insufficient, the toner remains on the downstream side of the nip portion in the rotating direction of the photosensitive drum, and defective printing such as hazy printing occurs.
Therefore, to enable an ideal transfer voltage to be applied, just before the print medium reaches the nip portion, a small potential difference (for example, about 1 kV) which does not damage the photosensitive drum is applied as a pre-voltage to an interval between the transfer roller and the photosensitive drum, and a current which is generated in association with the applied potential difference is read out. A resistance value of the transfer roller is calculated on the basis of the read-out current and fed back, thereby calculating an optimum transfer voltage (for example, 5000 V) with reference to a control table which has previously been formed.
The transfer roller is constructed by an axis made of a metal and an elastic layer formed around the axis. It is ideal that a resistance value between the axis and the surface of the transfer roller is set to a value within a range from 107 to 109Ω. The elastic layer is made of a foaming material using urethane, NBR, EPDM, silicone, or the like as a base material. Since each of the above materials inherently has insulation performance, the semiconductive roller whose resistance value has a proper value is molded by adding an electron conductive material such as carbon black, conductive polymer, metal filler, or the like or an ion conductive material according to an electrolyte into each of the above materials.
In the semiconductive roller, as methods of the electric conduction in the elastic layer, there are electron conduction by an electron conductive material and ion conduction by an ion conductive material. Electrical characteristics of the electron conduction and the ion conduction can be classified as shown in the Table 1. The motion of electrons, ions, and the like can be explained in accordance with a microscopic physical law and statistical law.
TABLE 1Electrical characteristicsElectronThe resistance value depends largely (exponentially)conductionon transfer voltageThe resistance value is constant without depending onthe temperature and humidityIonThe transfer current is directly proportional to theconductiontransfer voltage (the resistance value is constantwithout depending on the transfer voltage)The resistance value largely depends on temperatureand humidity
That is, in the electron conduction, although a resistance value depends largely (exponentially) on a transfer voltage, it is constant without depending on the temperature and the humidity. In the ion conduction, a generated transfer current is directly proportional to the transfer voltage (a resistance value is constant without depending on the voltage) and the resistance value depends largely on the temperature and the humidity.
Therefore, in the image forming apparatus, a transfer control program is adjusted in consideration of the electrical characteristics shown in Table 1.
FIG. 2 is a graph showing a relation between the transfer voltage and the transfer current in the conventional transfer roller. FIG. 3 is a graph showing a relation between the transfer voltage and a voltage margin in the conventional transfer roller. FIG. 4 is a graph showing a change in resistance value in association with changes in temperature and humidity in the conventional transfer roller. In FIG. 2, an axis of abscissa indicates the transfer voltage which is applied to the transfer roller and an axis of ordinate indicates the transfer current flowing in the transfer roller. In FIG. 3, an axis of abscissa indicates the transfer voltage and an axis of ordinate indicates the voltage margin. In FIG. 4, an axis of abscissa indicates states of environmental degrees of the temperature and the humidity and an axis of ordinate indicates a ratio of the resistance value at the time when the transfer roller is held in an environment of a high temperature and a high humidity and a ratio of the resistance value at the time when the transfer roller is held in an environment of a low temperature and a low humidity on the assumption that the resistance value which is obtained when the transfer roller is held in an environment of the normal temperature and the normal humidity is set to 1.00.
In FIG. 2, L1 denotes a line showing a relation between the transfer voltage and the transfer current in the electron conduction and L2 indicates a line showing a relation between the transfer voltage and the transfer current in the ion conduction, respectively. In FIG. 3, L3 denotes a line showing a relation between the transfer voltage and the voltage margin in the electron conduction and L4 indicates a line showing a relation between the transfer voltage and the voltage margin in the ion conduction, respectively.
In the electron conduction, since the transfer current is expressed by an exponential function of the transfer voltage as shown by the line L1, it is necessary to set the transfer voltage into an extremely narrow range in order to generate a predetermined transfer current. For example, if it is intended to generate the transfer current of 25±5 μA, it is sufficient to set the transfer voltage into a range from 1100 to 1600 V in the ion conduction, while it is necessary to set the transfer voltage into a range from 1000 to 1100 V in the electron conduction. In the electron conduction, since the resistance value depends largely on the transfer voltage, the voltage margin at the time when the transfer voltage is changed changes largely as shown by the line L3. On the other hand, the resistance value does not depend on the transfer voltage in the ion conduction. Therefore, the voltage margin at the time when the transfer voltage is changed is almost constant as shown by the line L4. The voltage margin shows a change μA/V in transfer current to the change in transfer voltage.
Since the resistance value of the transfer roller has a variation in the circumferential direction, for example, if the current deviated from an average value in the circumferential direction is read at a point when a pre-voltage is applied, the transfer voltage is not optimum and the transfer current which is generated is not optimum, either. When the transfer current is large, the transfer becomes excessive and, as mentioned above, the toner on the photosensitive drum is moved at a position just before the position on the upstream side of the nip portion in the conveying direction of the print medium and the defective printing such as what is called dust printing occurs. On the contrary, if the transfer current is small, the transfer is insufficient, the toner remains on the downstream side of the nip portion in the rotating direction of the photosensitive drum, and defective printing such as hazy printing occurs.
The higher a printing speed is, the shorter a transfer time becomes. It is, consequently, necessary to increase the transfer current. However, in the electron conduction, the range of the transfer voltage for optimally and generating the just enough transfer current is further narrowed.
In the ion conduction, the transfer current is expressed by a linear function of the transfer voltage as shown by the line L2 and the resistance value and an electric conductivity do not depend on the voltage. Therefore, since the transfer current can be precisely controlled better than that in the electron conduction, high picture quality can be realized.
However, as shown in FIG. 4, the ratio of the resistance value when the transfer roller is held in the environment of the low temperature and the low humidity to the resistance value when the transfer roller is held in the environment of the normal temperature and the normal humidity is equal to 2.07 in the case of the electron conduction and is equal to 5.23 in the case of the ion conduction. The ratio of the resistance value when the transfer roller is held in the environment of the high temperature and the high humidity to the resistance value when the transfer roller is held in the environment of the normal temperature and the normal humidity is equal to 1.36 in the case of the electron conduction and is equal to 0.10 in the case of the ion conduction, so that the resistance value fluctuates largely in dependence on the temperature and the humidity. In other words, the resistance value decreases in the environment of the high temperature and the high humidity, while the resistance value increases in the environment of the low temperature and the low humidity.
However, in the above conventional image forming apparatus, as shown in Table 2, there are the following problems based on the electric characteristics in both cases of the electron conduction and the ion conduction.
TABLE 2ProblemsElectronIt is diflicult to predict the transfer voltage and anconductionerror is likely to occur in the generated transfervoltageIon conductionA power source of a large capacity is necessary toobtain the necessary transfer current at the lowtemperature and low humidity
That is, in the case of the electron conduction, it is difficult to predict the transfer voltage and an error is likely to occur in the generated transfer voltage. In the case of the ion conduction, a power source of a large capacity is needed in order to obtain the transfer current necessary in the environment of the low temperature and the low humidity.
Therefore, in order to generate the optimum transfer current in any environment, the transfer voltage according to the resistance value is necessary. Particularly, if the resistance value increases when the transfer roller is held in the environment of the low temperature and the low humidity, it is necessary to raise the transfer voltage.
The higher the printing speed is, the shorter the transfer time becomes. It is, consequently, necessary to increase the transfer current. However, to generate the proper transfer current, it is necessary to raise the transfer voltage. In this case, since it is necessary to raise the transfer voltage in the environment of the low temperature and the low humidity, the power source of the large capacity is needed and costs of the power source rise.